Potential alternative to using rainbow crack as an external tool?

[NCrusher74]

I'm trying to develop an app that will allow people to convert the format of their own audiobooks to their preferred format. ffmpeg allows this if you have the activation bytes for your account. And getting those bytes would require me to add either a python tool for retrieving them online (which may stop working at any given time) or using a version of RainbowCrack-NG created for the purpose.

Either way, I'd have to add a command-line tool to my app's bundle and use it as an external tool (with or without the added complication of trying to make a Python script work with Swift) and I'd rather not do that if there's a Swift-based alternative.

Which I'm hoping CryptoSwift will be.

The problem is, I'm not all that familiar with the decryption side of things. From what I understand, it's fairly straightforward? There's a 40-byte SHA1 checksum that ffmpeg will detect. You hash that with the special version of RCrack and it returns your 8-byte activation key. But that's about as far as my understanding goes.

It looks like this is the code that is patched into RainbowCrack-NG for getting these activation bytes:

+void HashAudibleAAX(unsigned char* pPlain, int nPlainLen, unsigned char* pHash)
+{
+	unsigned char fixed_key[] = { 0x77, 0x21, 0x4d, 0x4b, 0x19, 0x6a, 0x87, 0xcd, 0x52, 0x00, 0x45, 0xfd, 0x20, 0xa5, 0x1d, 0x67 };
+
+	unsigned char hash1[20] = {0};
+	unsigned char hash2[20] = {0};
+	SHA_CTX ctx;
+
+	// fixed_key + "guess"
+	SHA1_Init(&ctx);
+	SHA1_Update(&ctx, fixed_key, 16);
+	SHA1_Update(&ctx, pPlain, nPlainLen);
+	SHA1_Final(hash1, &ctx);
+
+	// fixed_key + previous hash + guess
+	SHA1_Init(&ctx);
+	SHA1_Update(&ctx, fixed_key, 16);
+	SHA1_Update(&ctx, hash1, 20);
+	SHA1_Update(&ctx, pPlain, nPlainLen);
+	SHA1_Final(hash2, &ctx);
+
+	// final checksum calculation
+	SHA1_Init(&ctx);
+	SHA1_Update(&ctx, hash1, 16); // only 16 bytes!
+	SHA1_Update(&ctx, hash2, 16);
+	SHA1_Final(pHash, &ctx);
+}
+

But I could be missing something else since, as I said, I don't really understand how decryption and hashing works.

Is it possible to use CryptoSwift for this purpose, and if so, how would I do that?